1. Field of the Invention
The present invention relates to polyimide resin and particularly relates to a thermally stable polyimide and a process of preparing the polyimide.
2. Description of the Related Art
Conventionally, polyimide obtained by reaction of tetracarboxylic acid dianhydride with diamine is excellent in mechanical strength and dimensional stability in addition to its high heat resistance and flame retardance and electrical insulation property.
Because of these preferred properties, polyimide has been used in various fields such as electric and electronic appliance, space and aeronautic equipment, and transfer machinery. Polyimide is hence expected to be widely used in fields where heat resistance is required.
For example, polyimide having a fundamental skeleton of the following formula (V): ##STR3## has been widely used in the form of films and molded materials due to its excellent heat resistance, and mechanical and electrical properties. However, polyimide has a disadvantage that continuous use of the polyimide for a long time at high temperature in air leads to gradual reduction of these excellent properties and impairs essential properties of the polyimide.
Attempts have also been carried out to improve properties by blocking the polymer terminal of the polyimide having the fundamental skeleton of the formula (V). For example, Japanese Laid-Open Patent HEI 2-60931 and 2-60932 (1990) have disclosed polyimide and its precursor polyamic acid which have been blocked with a monoamine such as p-fluoroaniline in order to improve alkali resistance. U.S. Pat. No. 4,742,153 has also disclosed blocking the polyamic acid precursor of the formula (V) with pyromellitic anhydride. The invention, however, relates to a process for carrying out, in a drying step after blocking, solvent removal, imidization and simultaneous high polymer formulation of polyimide by intermolecular reaction of terminal carboxylic acid and terminal amine. That is, the invention relates to thermosetting polyimide.
Consequently, it has not been investigated to block the polymer terminal of polyimide having the formula (V) with dicarboxylic acid anhydride in order to improve thermal stability at high temperature.